The vertebrate skeleton is almost entirely mesodermal in origin, aside from several bones in the skull which are derived from neural crest (Noden, 1992; Couly et al., 1993). In Drosophila one of the principal genes controlling mesoderm differentiation is the bagpipe homeobox gene (Azpiazu and Frasch, 1993; Azpiazu et al., 1996). We have previously isolated from mouse and human, homologues of bagpipe termed Bapx1. Examination of the expression of Bapx1 during embryogenesis revealed an expression almost exclusively restricted to paraxial and lateral plate mesoderm, with earliest expression detectable in the presclerotome cells of the somite (Tribioli et al., 1997; Tribioli and Lufkin, 1997). During subsequent stages of embryogenesis, Bapx1 is expressed in essentially all cartilaginous condensations which will subsequently undergo endochondral ossification. To investigate the role of Bapx1 in embryonic skeletogenesis, this proposal focuses primarily on loss-of-function (gene knockout) and gain-of-function studies in vitro and in vivo. Preliminary results from mice lacking Bapx1 indicate that it plays an essential role in chondroblast differentiation within the axial skeleton, which in turn leads to failed endochondral ossification.